## Abstract

Quantum mechanical (QM) + molecular mechanics (MM) models are developed to represent potential energy surfaces (PESs) for the HBr^{+} + CO_{2} → Br + HOCO^{+} reaction with HBr^{+} in the ^{2}Π_{3/2} and ^{2}Π_{1/2} spin-orbit states. The QM component is the spin-free PES and spin-orbit coupling for each state is represented by a MM-like analytic potential fit to spin-orbit electronic structure calculations. Coupled-cluster single double and perturbative triple excitation (CCSD(T)) calculations are performed to obtain "benchmark" reaction energies without spin-orbit coupling. With zero-point energies removed, the "experimental" reaction energy is 44 ± 5 meV for HBr^{+}(^{2}Π_{3/2}) + CO_{2} → Br(^{2}P_{3/2}) + HOCO^{+}, while the CCSD(T) value with spin-orbit effects included is 87 meV. Electronic structure calculations were performed to determine properties of the BrHOCO^{+} reaction intermediate and [HBr⋯OCO]^{+} van der Waals intermediate. The results of different electronic structure methods were compared with those obtained with CCSD(T), and UMP2/cc-pVTZ/PP was found to be a practical and accurate QM method to use in QM/MM direct dynamics simulations. The spin-orbit coupling calculations show that the spin-free QM PES gives a quite good representation of the shape of the PES originated by ^{2}Π_{3/2}HBr^{+}. This is also the case for the reactant region of the PES for ^{2}Π_{1/2} HBr^{+}, but spin-orbit coupling effects are important for the exit-channel region of this PES. A MM model was developed to represent these effects, which were combined with the spin-free QM PES.

Original language | English |
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Article number | 104302 |

Journal | Journal of Chemical Physics |

Volume | 142 |

Issue number | 10 |

DOIs | |

State | Published - Mar 14 2015 |